A conventional electrochromic device enables to modify the optical transmittance, reflectance, or absorbance properties of an electromagnetic radiation. Such a modulation is obtained by means of an active electrochromic electrode, which allows a reversible change of optical state when an electric voltage is applied across the electrochromic device.
This type of device comprises a stack formed of the following elements:
a first current collector;
a first electrochromic electrode;
an electrolyte;
a second electrochromic electrode;
a second current collector; and
a support.
The current collectors are used as terminals enabling to connect the device to an external electric voltage source.
Further, at least one of the electrodes is optically active. It enables to modulate the optical properties of an electromagnetic radiation by passing from a colored state to a transparent state. Such a change of optical state results from the insertion or from the deinsertion of ions into or from the material forming the electrochromic electrode when an electric voltage is applied across the device.
The second electrode may also be optically active, but with a reverse ion flow, or passive, that is, transparent whatever the ion flow. It is necessary in known structures to provide the storage of metal ions which migrate from the first electrochromic electrode.
The electrolyte is an electronic insulator but an ion conductor. It enables to ensure the ion migration between the two electrodes.
The performance of an electrochromic device especially depends on the contrast, that is, on the difference between the maximum and the minimum optical response of the device. A high-performance device generally has a high contrast.
Even though prior art devices generally enable to obtain a satisfactory contrast, they undergo optical losses due to the absorption of the materials used and to the reflection phenomenon at the level of the substrate/current collector/electrochromic electrode/electrolyte/electrochromic electrode/current collector interfaces.
A solution enabling to limit optical losses, and thus to obtain a better contrast, comprises placing the active electrochromic electrode at the front surface of the device, that is, in direct contact with the electromagnetic radiation.
However, since the number of optical interfaces remains unchanged, such devices do not do away with optical losses induced by the interfaces. Further, such devices may also raise problems of cyclability in time as the cation insertion/deinsertion cycles are repeated. Document U.S. Pat. No. 7,042,615 describes a device of this type.
The present disclosure relates to an electrochromic device enabling to solve these technical problems, and especially to improve the contrast by decreasing optical losses.